Method and means for regulating the tensioning of workpieces in multi-pass cold-drawing apparatus

ABSTRACT

The tensioning of elongated tubular workpieces during transport through a multi-pass cold-drawing apparatus is regulated by changing the speed of discrete electric motors which drive work advancing drums located downstream of successive reducing dies. The speed of the motors is regulated in response to electric signals which are produced by first pressure gauges serving to measure the peripheral forces of the drums and second pressure gauges which measure the retarding forces of the preceding reducing dies. The evaluating circuit employs discrete first and second amplifiers which are respectively connected with the first and second pressure gauges, differential amplifiers each of which receives signals from a first and a second amplifier, rated value selecting devices which furnish preselected signals of constant intensity, and signal comparing circuits which compare the preselected signals with signals furnished by the corresponding differential amplifiers and produce motor-adjusting signals.

United States Patent Mertens et a1.

[76] Inventors: Oswald Mertens, Neuhauser Str. 15, Paderborn; l'lelmut Schulz, Querweg 29, Schloss Neuhaus, both of Germany [22] Filed: June 28, 1972 [21] Appl. No.: 267,172

[30] Foreign Application Priority Data June 30, 1971 Germany 2132408 [52] US. Cl 72/8, 72/279, 72/288 [51] Int. Cl. B2lc 1/04, B2lc 1/12 [58] Field of Search 72/8, l0, l1, 19, 279, 72/280, 288, 289

[56] References Cited UNITED STATES PATENTS 2,708,024 5/1955 Bryden .1 72/279 2,220,048 10/1940 Mueller.... 72/279 3,593,558 7/1971 Sperduti... 72/289 2,205,213 6/1940 Lessman 72/289 2,699,864 l/l955 Hitchcock et al. 72/279 51 Mar. 26, 1974 Russell; 72/289 Blodgett..

Primary Examiner-Charles W. Lanham Assistant Examiner-M. J. Keenan Attorney, Agent, or Firm-Michael S. Striker [57] ABSTRACT .circuit employs discrete first and second amplifiers which are respectively connected with the first and second pressure gauges, differential amplifiers each of which receives signals from a first and a second amplifier, rated value selecting devices which furnish preselected signals of constant intensity, and signal comparing circuits which compare the preselected signals with signals furnished by the corresponding differential amplifiers and produce motor-adjusting signals.

12 Claims, '7 Drawing Figures PMENIEDmzsmn 31798939 sum 1 or 3 FIG. 2

PATENTED M826 I974 SHEU 2 I)? 3 FIG. 3

FIG. 5

PATENIEDMARZS 1974 35798339 SHEET 3 BF 3 METHOD AND MEANS FOR REGULATING THE TENSIONING OF WORKPIECES IN MULTI-PASS COLD-DRAWING APPARATUS BACKGROUND OF THE INVENTION I The present invention relates to a method and apparatus for cold-drawing elongated workpieces, especially metallic tubes, and more particularly to improvements in a method and apparatus for cold-drawing workpieces in a plurality of successive passes. Still more particularly, the invention relates to improvements in a method and apparatus for regulating the tensioning of elongated workpieces during travel through a series of reducing dies.

In presently known multiple-pass cold-drawing apparatus, the transport of elongated workpieces through a series of reducing dies presents serious problems, especially as concerns the uniformity of specific tensional stresses in each portion of a workpiece. Thus, it is difficult to synchronize the operation of presently known advancing devices which pull the workpiece through successive reducing dies in such a way that the workpiece is subjected to predictable elongation. Therefore, the presently known cold-drawing apparatus normally resort to so-called magazines for conveyed material which are installed between successive draw benches. This contributes to the initial and maintenance cost as well as to space requirements of the cold-drawing apparatus. Furthermore, the provision of magazines prevents the apparatus from taking full advantage of tensional stresses which develop in a workpiece during cold-drawing and which could be utilized to reduce the resistance offered by a moving workpiece to the passage through a reducing die. Moreover, the magazines normally store lengths of tubular or solid workpieces in convoluted form which entails undesirable flexing of the material of workpieces. Such flexing might not unduly affect the quality of wire or small-caliber metallic rods but is likely to have an adverse effect upon the characteristics of cold-drawn tubes.

It was already proposed to regulate the action of discrete work-advancing devices in a multi-pass colddrawing apparatus in dependency on the tension in selected portions of a moving workpiece. The results of measurements are utilized to regulate the action of advancing devices for the purpose of insuring that the tension of a moving workpiece remains constant at each of the reducing stations. As a rule, the presently employed tension measuring gauges employ dancer rolls which detect the tension indirectly by monitoring the magnitude of forces acting at right angles to the direction of transport of workpieces through successive reducing dies. The dancer rolls transmit signals to suitable transducers which generate proportional electrical control signals for regulating the operation of associated advancing devices. A drawback of such proposal is that a dancer roll is likely to adversely affect the appearance and/or the shape of thin-walled smalldiameter metallic tubes or like sensitive workpieces. Furthermore, the accuracy of the transducers is adversely affected by the flexing work which is performed by dancer rolls to subject the adjacent portions of workpieces to bending stresses.

SUMMARY OF THE INVENTION An object of the invention is to provide a novel and improved method of regulating the tensioning of elongated metallic workpieces which are subjected to a multi-pass cold-drawing treatment. I

Another object of theinvention is to provide a novel method of regulating the motors which are utilized in advancing devices for elongated metallic workpieces in multi-pass cold-drawing apparatus.

A further object of the invention is to provide a multi-pass cold-drawing apparatus with novel and improved means for regulating the speed of operation of advancing devices which transport elongated metallic workpieces through a series of reducing dies.

The method of the present invention is utilized to regulate tensional stresses upon elongated workpieces in multi-pass cold-drawing apparatus wherein a workpiece, particularly a 'metallic tube, is being drawn through a series of successive reducing dies by rotary advancing elements located downstream of the respective dies. The method comprises the steps of measuring the magnitude of advancing or tensioning force applied to the workpiece by each of the advancing elements and the magnitude of retarding force applied to the workpiece by each of the reducing dies, and untilizing the results of such measurements to regulate the speed of the advancing elements, preferably by regulating the speed of electric motors which are employed to drive the advancing elements. The measuring step preferably comprises producing electric signals whose intensity is proportional to the magnitude of measured forces.

The aforementioned utilizing step preferably comprises producing signals which represent the sum of signals indicating the magnitude of forces applied by the dies and the magnitude of forces applied by the advancing elements following such dies, and regulating the speed of the respective advancing elements as a function of the intensity of the thus obtained signals. The thus obtained signal for a next-following advancing element is preferably added to the signal representing the force applied by the advancing element preceding the next-following advancing element to insure that the speed of the preceding advancing element isregulated in dependency on all forces which act on the workpiece during travel toward the next-following advancing element.

The forces are preferably measured by suitable force measuring devices, such as pressure gauges, which are capable of producing electric signals indicative of the .magnitude of measured forces.

The novel features which are considered as characte'ristic of the invention are set forth in particular in the appended claims. The improved multi-pass colddrawing apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic partly elevational and partly sectional view of a combined draw bench and work advancing device forming part of the improved multi-pass FIG. 3 is an enlarged partly elevational and partly longitudinal vertical sectional view of the upper portion of the draw bench shown in FIG. 1;

FIG. 4 is a sectional view as seen in the direction of arrows from the line IV-IV of FIG. 3;

FIG. 5 is a sectional view as seen in the direction of arrows from the line V-V of FIG. 3;

FIG. 6 is a sectional view as seen in the direction of arrows from the line VIVI of FIG. 3; and

FIG. 7 is a diagram showing the evaluating circuit which regulates the speed of motors for the advancing elements of several successive advancing devices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 illustrate one of a battery of successive combined cold-drawing and advancing units in a multipass cold-drawing apparatus. The advancing device 4 of the illustrated unit comprises a rotary advancing element in the form of a hollow drum 6 having a cylindrical external surface. An elongated tubular metallic workpiece 2 forms at least one convolution around the drum 6 (one convolution shown in FIG. 1) so that it is drawn through the reducing die 3 of the associated draw bench 1 when the drum is driven in a clockwise direction, as viewed in 'FIG. 1. The drum 6 is. mounted on antifriction roller bearings 8 which surround the hub 7 of an output shaft 9 of a variable-speed electric motor 5. The hub 7 can be made integral with or is bolted or otherwise secured to the output shaft 9. The hollow drum 6 accommodates in its interior an electrical force measuring device here shwon as a pressure gauge or load cell 10. The latter is mounted on the hub 7 and includes a yieldable plunger 10a which engages an abutment or anvil ll of the drum 6 when the motor Sis on to rotate the output shaft 9 in a clockwise direction, as viewed in FIG. 1.- The pressure gauge 10 then produces an electric signal whose intensity is proportional to the peripheral advancing force of the drum 6. The intensity of signal is proportional to the resistance which the drum 6 offers to rotation with the output shaft 9 and to the extent to which the abutment 11 depresses the plunger 10a. The extent of such depression is proportional to the torque which the pressure gauge 10 transmits to the drum 6.

The draw bench 1 comprises a base 1a which also supports the advancing device 4 including the drum 6 and motor 5. The base la supports a frame 13 (see also FIGS. 3-6) which is pivotable about the axis of a vertical shaft 190 and about the axis of a horizontal shaft 19 carried by the shaft 19a. The shaft 19a is turnable in a bearing sleeve 20 of the base la. The front portion of the frame 13 is provided with a holder or head 12 which removably supports the reducing die 3. The shaft 19, 19a enable the die 3 to assume a position in which its axis coincides with the axis ofthe workpiece 2 passing through the draw bench l. The holder 12 constitutes a beam'which is pivotable in the frame 13, as at 14 (see FIG. 4) and its lower portion is coupled to the frame 13 by means of a second force measuring device or pressure gauge 16. The pressure gauges 10 and 16 may be of one of the types disclosed in the German Pat. No. 886,082 and No. 960,236 and in the German documents No. 1,010,290, No. 1,034,391 r and No.

. 1,041,276 laid open to public inspection. The pressure gauge 16 is pivoted to the holder 12 and frame 13, as

at 15 and 17 (see also FIGS. 3 and 6). The frame 13 is provided with a wheel 18 which can roll along a platform of the base la. This frame takes up the reaction forces which are produced by the pressure'gauge 16. The line of forces which develop in response to deformation ofthe workpiece 2 during travel through the die 3 is located between the pivot member 14-and pivot members l5, l7 and is parallel to the axis of the pressure gauge 16. The shaft can be moved up or down by an adjusting device 19!), e.g., a feed screw. The electric evaluating circuit which regulates the speed of the motors 5 of two, three ormore (n) successive advancing devices 4 for the workpiece 2 is shown in FIG. 7.

The workpiece 2 is assumed to advance in the direction indicated by arrow A and is treated during travel through the dies 3 of a battery of draw benches 1, each located upstream of an advancing device 4. The motors S are d-c motors each having a field winding 21 and each thereof is associated with a tachometer generator 22 serving to furnish signals which indicate the RPM of the'respective output shaft 9. The evaluating circuit of FIG. 7 includes for each of the motors 5 a master switch 23, a braking resistor 24, a thyristor bridge 25, a current transformer 26, a filter choke 27, and a safety fuse 28. The reference character 29 denotes a bus bar for polyphase current. The'output signal of each pressure gauge 10 is transmitted to a discrete amplifier 30, and the output signal of each pressure gauge 16 is transmitted to a discrete amplifier 31. Each pair of amplifiers 30, 31 is connected to a discrete differential amplifier 32 which transmits signals to a signal comparing circuit 33 which further receives signals from a rated value selector 33a, e.g., an adjustable potentiometer. The output of each circuit 33 is connected with a junction 33b which further receives signals from the respective tachometer generator 22. A conductor 34 connects a common starter integrator (not shown) with the junctions 33b by way of adjustable potentiometers 35 for forward transport. The evaluating circuit further comprises RPM regulators 36, current regulators 3,7 and pulse shapers 38.

In operation, the drums 6 receive torque from the respective motors 5 by way of the associated pressure gauges 10. The bus bar 29 supplies polyphase alternating current whichis conducted to the d-c motors 5 byway of respective fuses 28, chokes 27, thyristor bridges 25 which act as rectifiers, and closed master switches 23. The field windings 21 are supplied with constant current. On opening of the master switches 23, the braking resistors 24 are connected in circuit with the respective motors 5.

The evaluating circuit of FIG. 7 regulates the speed of the motors 5 by limiting the current with superimposed regulation of tension in the workpiece 2.

The current limiting operation is carried out as follows: The tachometer generators 22 furnish voltage signals which are proportional to the RPM. of the respective motors 5. The adjustable potentiometers 35 are set to furnish constant signals which are indicative of the selected RPM for the corresponding drum 6. The potentiometers 35 receive current from the conductor 34 which is connected to a start integrator (not shown) common to all advancing devices'4. The value of signals furnished by the tachometer generators 22 is deducted from the signals furnished by the corresponding potentiometers 35, and the voltage differential is applied to the RPM regulators 36. The latter are connected with the corresponding current regulators 37 which control the pulse shapers 38. The pulses furnished by 38 determine the current which is allowed to pass by the thyristor bridges 25. The current transform ers 26 furnish voltages which are proportional to the armature current of the respective motors 5. Such voltages are applied to the current regulators 37 and serve to insure that the armature current of the motors 5 cannot exceed a predetermined maximum value.

The regulation of tension in the workpiece 2 is carried out as follows: During drawing, the pressure gauge in each advancing device 4 and the pressure gauge 16 in each draw bench 1 is subjected to certain stresses. The gauges furnish electrical signals which are indicative of the respective stresses. Such signals are amplified by the respective amplifiers 30, 31 and are transmitted to the corresponding differential amplifier 32.

The latter furnishes a signal which represents a differ-' ence of the signals received from the associated amplifiers 30, 31. The potentiometer 33a is adjusted to furnish a signal which is indicative of the selected difference between the signals furnished by the corresponding amplifiers 30, 31. Such signal is added to the output signal of the differential amplifier 32 and is transmitted to the input of the corresponding signal comparing circuit 33. The output signal of the circuit 33 is superimposed upon the signal furnishetl to the input of the respective RPM regulator 36.

The output signal of the differential amplifier 32 in a next-following advancing device (n in FIG. 7) is transmitted via conductor 50 to a third input of the differential amplifier 32 in the preceding advancing device 4 (n-i i it will be noted that the-apparatus of the present invention directly measures the magnitude of tensional stresses acting upon a workpiece 2 between and in successive draw benches 1 and regulates the magnitude of such stresses so as to maintain the stresses within a desired range. The regulation of stresses is carried out by changing the RPM of motors 5 in the advancing de vices 4. The monitoring or measuring means includes the pressure gauges 10 which furnish electrical signals proportional to forces which are produced by the drums 6, and the gauges 16 which furnish electrical signals indicating the magnitude of forces developing at the dies 3. These signals are utilized to regulate the speed of motors 5 with a view to insure that the tensional stresses upon the workpiece 2 remain within a selected range. The maintaining of tensional stresses upon the workpiece 2 at a constant value is a determining factor for satisfying the equation wherein P, is the tensional force upon the workpiece 2 at the upstream side of a drum 6, AP is the tensional force downstream of the drum 6, e is the natural base 2.17828, 0: is the angle of draping the workpiece 2 around the drum 6, and yis the friction coefficient between the workpiece and the drum. The above equation is known as Euler's formula relating to the angle a of looping a cable or the like around a pulley or drum.

The equation indicates that the drum 6 must furnish a peripheral force which exceeds the deforming force ofthe die 3, i.e., a force which acts upon the workpiece between two successive cold-drawing and advancing units and tends to drive the drum 6 in the preceding advancing device 4 and to thus relieve the preceding 6 advancing device. in accordance with the method of the present invention, the selected force AP for the corresponding draw bench can be maintained at a constant value during all stages of a cold-drawing operation. The signals furnished by differential amplifiers 32 are compared with signals from the correspondingpotentiometers 33a. When the intensity of signal furnished by a differential amplifier 32 increases, the respective motor 5 is decelerated, and vice versa. The antifriction bearings 8 reduce the effect of friction upon the signals which are furnished by the pressure gauges 10 when the motors'5 rotate to drive the respective drums 6 by way of the plunges 10a and abutments 11. Such arrangement insures that, by properly calibrating the amplifiers 30, the signals reaching the corresponding inputs of the differential amplifiers 32 are truly representative of the momentary peripheral forces furnished by the corresponding drums 6.

Each holder 12 is pivotable relative to the corresponding frame 13, as'at 14, and can act upon the associated pressure gauge 16 by way of the pivot 15. The reaction force furnished by the pressure gauge 16 is taken up by the respective frame 13 through theintermediary of the pivot 17. The direction of deforming forces acting upon the workpiece 2 during travel through the die 3 is parallel to the axis of the pressure gauge 16, and such force acts between the pivots 14, 15. By properly calibrating the pressure gauge 16 in dependency on the leverage of the holder 12, the signal furnished by the pressure gauge 16 indicates, with great accuracy, the magnitude of forces acting on the die 3. Such signal can be read on the dial which is preferably provided on each pressure gauge 16. t

The potentiometers 33a furnish signals which represent the selected advancing forces to be furnished by the drums 6. The voltages furnished by differential amplifiers 32 represent the actual advancing forces and are-compared with the signals from the respective potentiometers 33a. Consequently, the signal from acirsuit 33 is indicative of positive or negative difference between the signals from 32 and 33a, and such signal is used to reduce the speed of the corresponding motor 5 when the intensity of signal from the amplifier 32 exceeds that of the signal from the potentiometer 33a, or vice versa.

The differential force AP equals the advancing force furnished by a drum 6 minus the force furnished by the next-following die 3. Such differential force acts as a driving force upon the preceding drum 6. Therefore, the output signal from the differential amplifier 32in a nextfollowing advancing device 4 is preferably added to the signal furnished by the pressure gauge 10 in the preceding advancing device 4 (see the conductor 50 in FIG. 7) to thus insurethat the signal furnished. by the output of the amplifier 32 in the preceding advancing device 4 is truly representative of all forces acting on the workpiece 2. The conductor 50 can be omitted if the workpiece 2 is capable of withstanding the higher stresses which act thereupon in the preceding advancing device. l

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of our contribution to the art and. therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

We claim:

1. A method of regulating the tensional stresses upon elongated workpieces in multi-pass cold-drawing apparatus wherein a workpiece, particularly a tubular workpiece, is being drawn through a series of successive reducing dies by rotary advancing elements. located downstream of the respective dies, comprising the steps of directly measuring the magnitude of forces applied to the workpiece by each of the advancing elements and by each of the dies, and producing electric signals whose intensity is proportional to the magnitude of measured forces; and utilizing the results of such direct measurements to regulate the speed of the advancing elements, by producing signals which represent the sum of signals indicating the magnitude of forces applied by the dies and the magnitude of forces applied by the advancing elements following such dies, and regulating the speed of the respective advancing elements as a function of the intensity of the thus obtained signals.

2. A method as defined in claim 1, further comprising the step of adding the thus obtained signal for a nextfollowing advancing element to the signal representing the force applied by the advancing element preceding such next-following advancing element.

3. In a multi-pass cold-drawing apparatus for elongated workpieces, particularly tubular workpieces, a combination comprising successive first and second draw benches each including a reducing die; successive first and second advancing means respectively located downstream of said first and second draw benches and each including a rotary advancing element arranged to draw on elongated workpiece through the preceding reducing die and a variable-speed electric DC motor for driving the respective advancing element; and means for regulating the speed of said motors, including first force measuring devices associated with said advancing elements and arranged to produce first electric signals indicative of the peripheral forces applied by the respective advancing elements to an advancing workpiece, second force measuring devices associated with said dies and arranged to produce second electric signals indicative of retarding forces applied to the advancing workpiece by the respective dies, and evaluating means arranged to regulate the speed of said motors as a function of the intensity of first signals furnished by the respective first force measuring devices and second signals furnished by the respective second force measuring devices, said evacuating means comprising first and second differential amplifiers each connected with a first force measuring device and with the associated second force measuring device and arranged to furnish signal representing the sum of the thus received signals, rated value selectors arranged to furnish signals of selected constant intensity, and signal comparing means arranged to furnish motor-adjusting signals representing the difference between the intensities of signais furnished by a differential amplifier and the corresponding constant-intensity signal so that the speed of said motors decreases when the intensity of signals furnished by, the respective differential amplifiers increases and vice versa/ 4 A combination as defined in claim 3, wherein said first and second force measuring devices are first and second pressure gauges.

5. A combination as defined in claim 4, wherein each of said motors has a rotary output member and each of .said advancing elements is rotatable on the respective means located in the path of movement of the respective first pressure gauge.

7. A combination as defined in claim 6, wherein each of said first pressure gauges includes a plunger which bears against the respective abutment means.

8. A combination as defined in claim 5, further comprising antifriction bearing means interposed between said output members and the respective advancing elements.- 7

9. A combination as defined in claim 3, wherein each of said draw benches further comprises a frame and a die holder pivoted to said frame, said second force measuring devices being articulately connected between said frames and the respective holders,

10. A combination as defined in claim 9, wherein said second force measuring devices are pressure gauges which are at least substantially parallel to the axes of reducing dies in the respective holders.

1]. in a multi-pass cold-drawing apparatus for elongated workpieces, particularly tubular workpieces, a

combination comprising successive first and second draw benches each including a reducing die; successive first and second advancing means respectively located downstream of said first and second draw benches and each including a rotary advancing element arranged to draw on elongated workpiece through the preceding reducing die and a variable-speed electric motor for driving the respective advancing element; and means for regulating the speed of said motors, including first force measuring devices associated with said advancing elements and arranged to produce first electric signals indicative of the peripheral forces applied by the respective advancing elements to an advancing workpiece, second force measuring devices associated with said dies and arranged to produce second electric signals indicative of retarding forces applied to the advancing workpiece by the respective dies, and evaluating means arranged to regulate the speed of said motors as a function of the intensity of first signals furnished by the respective first force measuring devices and second signals furnished by the respective second force measuring devices, said evaluating means comprising discrete signal amplifiers electrically connected with said force measuring devices and differential amplifiers each electrically connected with the signal amplifier for a first force measuring device and the signal amplifier for the associated second force measuring device.

12. A combination as defined in claim 3, wherein said evaluating means produces third signals representing the sum of said first and second signals; and further comprising meansfor adding said third signals for a next-following advancing means to the first signal indicative of the forces applied to the advancing workpiece by the preceding advancing element.

l t i t i 

1. A method of regulating the tensional stresses upon elongated workpieces in multi-pass cold-drawing apparatus wherein a workpiece, particularly a tubular workpiece, is being drawn through a series of successive reducing dies by rotary advancing elements located downstream of the respective dies, comprising the steps of directly measuring the magnitude of forces applied to the workpiece by each of the advancing elements and by each of the dies, and producing electric signals whose intensity is proportional to the magnitude of measured forces; and utilizing the results of such direct measurements to regulate the speed of the advancing elements, by producing signals which represent the sum of signals indicating the magnitude of forces applied by the dies and the magnitude of forces applied by the advancing elements following such dies, and regulating the speed of the respective advancing elements as a function of the intensity of the thus obtained signals.
 2. A method as defined in claim 1, further comprising the step of adding the thus obtained signal for a next-following advancing element to the signal representing the force applied by the advancing element preceding such next-following advancing element.
 3. In a multi-pass cold-drawing apparatus for elongated workpieces, particularly tubular workpieces, a combination comprising successive first and second draw benches each including a reducing die; successive first and second advancing means respectively located downstream of said first and second draw benches and each including a rotary advancing element arranged to draw on elongated workpiece through the preceding reducing die and a variable-speed electric DC motor for driving the respective advancing element; and means for regulating the speed of said motors, including first force measuring devices associated with said advancing elements and arranged to produce first electric signals indicative of the peripheral forces applied by the respective advancing elements to an advancing workpiece, second force measuring devices associated with said dies and arranged to produce second electric signals indicative of retarding forces applied to the advancing workpiece by the respective dies, and evaluating means arranged to regulate the speed of said motors as a function of the intensity of first signals furnished by the respective first force measuring devices and second signals furnished by the respective second force measuring devices, said evacuating means comprising first and second differential amplifiers each connected with a first force measuring device and with the associated second force measuring device and arranged to furnish signal representing the sum of the thus received signals, rated value selectors arranged to furnish signals of selected constant intensity, and signal comparing means arranged to furnish motor-adjusting signals representing the difference between the intensities of signals furnished by a differential amplifier and the corresponding constant-intensity signal so that the speed of said motors decreases when the intensity of signals furnished by the respective differential amplifiers increases and vice versa.
 4. A combination as defined in claim 3, wherein said first and second force measuring devices are first and second pressure gauges.
 5. A combination as defined in claim 4, wherein each of said motors has a rotary output member and each of said advancing elements is rotatable on the respective output member, said first pressure gauges being arranged to transmit torque from said output members directly to the respective advancing elements.
 6. A combination as defined in claim 5, wherein each of said output members has a hub supporting the respective first pressure gauge and each of said advancing elements is a hollow drum having internal abutment means located in the path of movement of the respective first pressure gauge.
 7. A combination as defined in claim 6, wherein each of said first pressure gauges includes a plunger which bears against the respective abutment means.
 8. A combination as defined in claim 5, further comprising antifriction bearing means interposed between said output members and the respective advancing elements.
 9. A combination as defined in claim 3, wherein each of said draw benches further comprises a frame and a die holder pivoted to said frame, said second force measuring devices being articulately connected between said frames and the respective holders.
 10. A combination as defined in claim 9, wherein said second force measuring devices are pressure gauges which are at least substantially parallel to the axes of reducing dies in the respective holders.
 11. In a multi-pass cold-drawing apparatus for elongated workpieces, particularly tubular workpieces, a combination comprising successive first and second draw benches each including a reducing die; successive first and second advancing means respectively located downstream of said first and second draw benches and each including a rotary advancing element arranged to draw on elongated workpiece through the preceding reducing die and a variable-speed electric motor for driving the respective advancing element; and means for regulating the speed of said motors, including first force measuring devices associated with said advancing elements and arranged to produce first electric signals indicative oF the peripheral forces applied by the respective advancing elements to an advancing workpiece, second force measuring devices associated with said dies and arranged to produce second electric signals indicative of retarding forces applied to the advancing workpiece by the respective dies, and evaluating means arranged to regulate the speed of said motors as a function of the intensity of first signals furnished by the respective first force measuring devices and second signals furnished by the respective second force measuring devices, said evaluating means comprising discrete signal amplifiers electrically connected with said force measuring devices and differential amplifiers each electrically connected with the signal amplifier for a first force measuring device and the signal amplifier for the associated second force measuring device.
 12. A combination as defined in claim 3, wherein said evaluating means produces third signals representing the sum of said first and second signals; and further comprising means for adding said third signals for a next-following advancing means to the first signal indicative of the forces applied to the advancing workpiece by the preceding advancing element. 